The present invention relates generally to instrumentation amplifiers, and more particularly to circuitry and techniques for providing “seamless” or continuous circuit operation between positive and negative portions of a common mode input range.
The closest prior art is believed to be the assignee's INA 168 positive common mode voltage instrumentation amplifier, a diagram of which is shown in FIG. 1. Referring to FIG. 1, prior art instrumentation amplifier 1 has positive common mode voltage operation between a single supply voltage V+ and ground. Amplifier 1 includes an operational amplifier 7A having a (+) input coupled by a conductor 12 to one terminal of a precision input resistor 5, which may, for example, have a resistance Rin equal to 5 kilohms. The other terminal of resistor 5 is coupled by a conductor 2 to receive an input voltage Vin+ and also is connected to one terminal of a sense resistor Rs, the other terminal of which is connected by a conductor 3 to one terminal of an external load 4, the other terminal of which is connected to ground. Sense resistor RS has a very low resistance, typically 10 milliohms, and a current IS flows through sense resistor RS into load 4 in response to a positive value of Vin+. The current IS develops a small sense voltage across sense resistor RS. Conductor 3 is connected to one terminal of another precision input resistor 6, which also has the same resistance Rin. The other terminal of resistor 6 is connected to the (−) input of operational amplifier 7A. Resistor 6 is precisely matched to resistor 5.
The output of operational amplifier 7A is connected by a conductor 10A to the base of an NPN transistor Q1, the emitter of which is connected by an output conductor 11A to one terminal of a load resistor RL, the other terminal of which is connected to ground. The collector of transistor Q1 is connected by conductor 12 to provide feedback to the (+) input of operational amplifier 7A. Positive common mode voltage instrumentation amplifier 1 thus precisely measures the voltage across sense resistor Rs and produces an output voltage Vout on output conductor 11A, where Vout=(IS)(RS)(RL)/Rin. Thus, Vout accurately represents the current IS supplied to load 4 by Vin+.
The resistance of sense resistor RS is small, typically approximately 10 milliohms to 1 ohm, depending on the value of IS. The voltage on either conductor 2 or conductor 3 of FIG. 1 can be considered to be the positive common mode voltage. The supply voltage V+ typically has a value in the range from a few volts up to approximately +18 volts. Vin+ can be any value from roughly 1.4 volts to roughly 60 volts. The feedback provided by transistor Q1 keeps the voltage difference between the (+) and (−) inputs of operational amplifier 7A at nearly zero, and the current Iin2 through resistor 6, and hence the resulting voltage drop across it, are essentially zero. Therefore, the voltage across resistor Rs is equal to the voltage developed across 5 kilohm resistor 5. This voltage across resistor 5 produces a current Iin1 that flows through transistor Q1 and resistor RL.
The common mode voltage on conductor 3 must be positive, because amplifier 7A will turn transistor Q1 off, causing positive common mode voltage instrumentation amplifier 1 to be inoperative if Vin+ is below ground.
There are various applications of positive common mode voltage instrumentation amplifier 1 of prior art FIG. 1, including automotive applications in which both positive and negative transient voltages may occur. However, in such applications, positive common mode voltage instrumentation amplifier 1 may produce undesirable “glitches” in the output voltage Vout.
Thus, there is an unmet need for an instrumentation amplifier which has a wide common mode input voltage range in order to effectively reject both positive and negative transient voltage spikes.
There also is an unmet need for such an instrumentation amplifier that also provides a low output impedance.
There also is an unmet need for an instrumentation amplifier which has a wide common mode input voltage range and which avoids producing abrupt changes and/or inaccuracies in its output signal if the common mode input voltage applied to the instrumentation amplifier undergoes a transition from a positive to a negative value or vice versa.